1. Field of the Invention
This invention relates to positive end expiratory pressure (PEEP) control in a piston ventilator for controlling the residual pressure in the patient's lungs during retraction through an exhalation valve through closed loop pressure support in an exhalation control system by adjusting a signal corresponding to pneumatic pressure delivered to an exhalation valve in the patient circuit. The closed loop PEEP control maintains a pressure in the patient circuit during the expiratory phase of the breath by adjusting the pneumatic pressure on the exhalation valve as it relates to the pneumatic pressure in the patient circuit. More particularly, it is concerned with a system using a PEEP control valve which selectively vents gas to control the pressure in a signal line coupled with the exhalation valve, the opening of the PEEP control valve being dependent on a PEEP signal detected by a sensor in the exhalation control system in communication with the patient circuit.
2. Description of the Prior Art
Ventilators of the type concerned herein are designed to supply air or other breathable gases to assist a patient in breathing, and particularly to provide pressurized air to aid patients requiring respiratory assistance. In some circumstances, mandatory breath patterns are supported by the ventilator, while in others, a spontaneous breath pattern is supported. In addition, it is often desirable to provide supplemental oxygen to enrich the oxygen content of the gas inhaled by the patient.
An oxygen concentrator having a reservoir for receiving oxygen-enriched gas, including a sensor for monitoring withdrawal of the enriched gas from the reservoir, and a microprocessor for determining the minimum time for charging of the gas to provide a product gas with a selected oxygen concentration at the sensed rate of withdrawal is shown in U.S. Pat. No. 4,561,287. Another system for mixing the oxygen with air or another gas in a ventilation system in predetermined proportions involves the use of separate inlets into a pressure vessel up to respective first and second pressures is described in U.S. Pat. Nos. 4,022,234 and 4,023,587. The system shown therein operates in alternating withdrawal and mixing cycles. A feedback control of the rate of flow and pressure of breathing gas to a patient by an inspiration servounit is described in U.S. Pat. No. 3,741,208. U.S. Pat. No. 5,383,449 provides for control of oxygen concentration in a breathable gas by calculation of the mole ratios and pressure in the containment vessel, and by sequentially injecting oxygen and another gas to desired pressure values. These so-called batch mixing ventilators represent one system for patient ventilation.
While such systems are very useful in hospitals and other health care facilities, smaller and more confined devices not requiring connection to pressurized air are often more appropriate for home care. Piston and bellows types of ventilators allow delivery of a predetermined volume of breathing gas at a desired pressure responsive to the initiation of inspiratory efforts by a patient. Piston based ventilators can typically be made to be more compact than bellows based ventilators, but piston ventilators typically blend pressurized air and oxygen in a high pressure blender. The resultant mixture is then drawn by a piston through a valve that reduces the pressure of the mixture. Such systems typically do not permit the use of room air and pressurized oxygen, and can result in some risk of overpressurization in the event of failure of a high pressure gas delivery valve controlling introduction of one of the breathing gas components into the high pressure blender.
Another system for blending oxygen in a ventilator is shown in International Publication No. WO 96/24402 published Aug. 15, 1996. This system is designed for mixing gases at approximately ambient atmospheric pressure, such as oxygen and air. The mixing apparatus includes a piston disposed within a pump chamber. A flow limiting inlet controls introduction of oxygen for mixing with air, and the pressure of the oxygen is limited to an acceptable maximum pressure whereby even if the oxygen valve fails, the breathing gas will not be provided at an excessive pressure. A demand valve is alternately provided for reducing the pressure of the oxygen supplied before mixing, and a pressure sensor is also provided downstream of the demand valve for detecting failure of the demand valve to shut off the supply of the oxygen to prevent overpressurization.
The positive end expiratory pressure (PEEP) is the pressure in the breathing circuit in close proximity to the patient during the exhalation phase of the breath. It is known to select a PEEP for ventilators operating in a spontaneous breath pattern, as set forth in U.S. Pat. No. 5,383,449. However, the use of the PEEP value in ventilators, particularly piston-type ventilators, has been limited to controlling the mode of operation, e.g. the exhalation valve being fully open for mandatory breath patterns or fully closed for spontaneous breath patterns.
It would therefore be desirable to provide an exhalation control system, particularly in a piston-type ventilator, which would permit regulation of the amount of restriction provided by the exhalation valve to promote or prompt the patient to take the next breath following at the end of exhalation and the commencement of inhalation.